Electrochemical energy storage system

ABSTRACT

An electrochemical energy storage system includes a positive electrode, a negative electrode disposed proximally to and not in contact with the positive electrode, and a non-aqueous electrolyte, wherein the positive electrode and the negative electrode are immersed in the non-aqueous electrolyte, and a case is presented in the energy storage system to accommodate the non-aqueous electrolyte, the positive electrode, and the negative electrode. The positive electrode has a porous matrix having a plurality of micrometer sized pores and nanostructured metal oxides, wherein the porous matrix is a 3-dimensional (3D) mesoporous metal or a 3D open-structured carbonaceous material, and the nanostructured metal oxides are coated inside the plurality of pores of the porous matrix. The non-aqueous electrolyte includes organic salts having acylamino group and lithium salts characterized as LiX, wherein Li is lithium and X comprises ClO 4   − , SCN − , PF 6   − , B(C 2 O 4 ) 2   − , N(SO 2 CF 3 ) 2   − , CF 3 SO 3   − , or the combination thereof.

The current application claims a priority to the U.S. 61/579,495 filedon Dec. 22, 2011.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an electrochemical energy storagesystem, and more particularly, to a supercapacitor comprisingnanostructured metal oxide deposited on metal foam or carbon paper as anelectrode and lithium-containing quasi-ionic liquid as electrolyte.

2. Background

Applications for electrochemical energy storage are expanding rapidly asdemand grows in various areas including green energy storage andelectric-powered transportation. Electrochemical capacitors (ECs) showgood potential for high-power applications, but have lower energydensity than lithium batteries. As both energy and power densities of asupercapacitor relate to the square of the operating voltage, anelectrolyte with a large potential domain of stability is crucial.Conventional aqueous electrolyte typically exhibits a potential domainof 1V, limiting its capacitor cell voltage. Non-aqueous electrolytessuch as organic solvents do not allow the cell to be operated at hightemperatures due to their volatile, flammable, and thermally andelectrochemically unstable nature.

The overall performance of a supercapacitor depends not only on theselection of electrolytes but also on the selection of electrodematerials. For application in a range of energy storage devices, MnO₂has been extensively investigated as a promising electrode materialbecause of its high energy density, low cost, minimal environmentalimpact, and natural abundance. ECs with MnO₂ films as negative electrodeand ionic liquid as electrolyte have been investigated and recorded invarious prior arts. The research discovered that the cations such asn-butyl-n-methylpyrrolidinium, 1-ethyl-3-methylimidazolium, and1-butyl-3-methyl-imidazolium adsorb only on the electrode surface and donot penetrate into the [MnO₆] octahedral framework. Moreover, innon-aqueous electrolytes, the variation of the oxidation state of Mn ina MnO₂ electrode is approximately 0.4, which is smaller than that (˜0.7)observed in aqueous electrolytes.

A small variation in the Mn oxidation state implies that a lowpercentage of Mn in the structures has undergone reduction-oxidation(redox) reaction, indicating a low rate of ion insertion. This conditionis also associated with low electronic and ionic conductivity of MnO₂,which kinetically limits the rapid faradaic redox reactions in the bulkmaterials.

Therefore, the development of a new electrolyte having a large potentialdomain and a high stability under high temperature is required, and anew electrode with properties that enhance penetration of an electrolyteto compensate the low redox reaction among the charge/discharge cycle isalso necessary. Essentially, the desired MnO₂ electrode should havebetter electronic and ionic conductivity. The present inventiondiscloses materials and specific structures to solve the above-mentionedproblems in order to improve the capacity performance of conventionalECs.

In the present invention, the maximum energy density of 300 to 450 W hkg⁻¹ obtained from 3D porous metal oxides as an electrode in ionicliquid electrolyte-based EC is notably higher than those of symmetrical(or asymmetrical) ECs based on grapheme ECs (2.8 to 136 W h kg⁻¹), MnO₂nanowire/grapheme composite (MNGC) ECs (30.4 W h kg⁻¹), activated carbonECs (<10 W h kg⁻¹), or MnO₂ nano spheres/carbon nanotubes/polymercomposite as an electrode. Moreover, the embodiment of the presentinvention exhibits a superior power density (˜60 kW kg⁻¹ at 70 to 120 Wh kg⁻¹) and acceptable cycling performance of ˜95% retention after 500cycles.

SUMMARY

One embodiment of the present invention discloses a non-aqueouselectrolyte for an electrochemical energy storage system, comprising:organic salts having acylamino group and lithium salts characterized asLiX, wherein Li is lithium and X comprises ClO₄ ⁻, SCN⁻, PF₆ ⁻, B(C₂O₄)₂⁻, N(SO₂CF₃)₂ ⁻, CF₃SO₃ ⁻, or the combination thereof.

Another embodiment of the present invention discloses an electrode of anelectrochemical energy storage system, comprising: a porous matrixhaving a plurality of micrometer sized pores and nanostructured metaloxides, wherein the porous matrix is a 3-dimensional (3D) mesoporousmetal or a 3D open-structured carbonaceous material, and thenanostructured metal oxides are coated inside the plurality of pores ofthe porous matrix.

Another embodiment of the present invention discloses an electrochemicalenergy storage system, comprising a positive electrode, a negativeelectrode disposed proximally to and not in contact with the positiveelectrode, and a non-aqueous electrolyte, wherein the positive electrodeand the negative electrode are immersed in the non-aqueous electrolyte,and a case is presented in the energy storage system to accommodate thenon-aqueous electrolyte, the positive electrode, and the negativeelectrode. The positive electrode has a porous matrix having a pluralityof micrometer sized pores and nanostructured metal oxides, wherein theporous matrix is a 3-dimensional (3D) mesoporous metal or a 3Dopen-structured carbonaceous material, and the nanostructured metaloxides are coated inside the plurality of pores of the porous matrix.The non-aqueous electrolyte includes organic salts having acylaminogroup and lithium salts characterized as LiX, wherein Li is lithium andX comprises ClO₄ ⁻, SCN⁻, PF₆ ⁻, B(C₂O₄)₂ ⁻, N(SO₂CF₃)₂ ⁻, CF₃SO₃ ⁻, orthe combination thereof.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the present invention are illustratedwith the following description and upon reference to the accompanyingdrawings in which:

FIG. 1 shows the reaction block diagram of the binary mixture used as anelectrolyte according to one embodiment of the present invention;

FIG. 2 lists the chemical formula of the organic molecules constitutingthe electrolyte according to one embodiment of the present invention;

FIG. 3 presents the reaction forming the electrolyte in chemical formulaaccording to one embodiment of the present invention;

FIG. 4 shows a scanning electron microscopy (SEM) image of nickel foamaccording to one embodiment of the present invention;

FIG. 5 shows a cyclic voltammogram (CV) of a blank nickel foam accordingto one embodiment of the present invention;

FIG. 6 displays an SEM image of MnO₂ nanowires electrodeposited on tothe surface of the nickel foam according to one embodiment of thepresent invention;

FIG. 7 shows a transmitted electron microscopy (TEM) image of MnO₂nanowires according to one embodiment of the present invention;

FIG. 8 shows cyclic voltammograms of MnO₂ nanowires on nickel foam(MNNF) in the aqueous electrolyte, the LiClO₄-urea ionic liquid, and theLiClO₄-OZO ionic liquid according to one embodiment of the presentinvention;

FIG. 9 shows a chronopotentiogram (CP) of the MNNF electrode forcharge-discharge cycles in an applied potential from −1.5V to +1.0V andan applied current density of ∓1.5 A g⁻¹ according to one embodiment ofthe present invention;

FIG. 10 shows a CP of the MNNF electrode for charge-discharge cycles inan applied potential from −2.1V to +2.1V and an applied current densityof ∓1.0 A g⁻¹ according to one embodiment of the present invention;

FIG. 11 shows a CP of the MNNF electrode for charge-discharge cycles inan applied potential from −1.8V to +1.2V and an applied current densityof ∓1.0 A g⁻¹ according to one embodiment of the present invention; and

FIG. 12 shows an electrochemical energy storage system having a sealedcontainer in one embodiment of the present invention.

DETAILED DESCRIPTION

One embodiment of the present invention discloses electrodepositednanostructured metal oxides on metal foam or carbon fiber paper aselectrodes coupled with novel Li-ion ionic liquids (IL) as electrolytes.As shown in FIG. 1, the IL in this embodiment was prepared bylithium-containing salts and organic salts, wherein the reactants (i.e.,lithium-containing salts and organic salts) are in solid state powderbefore the reaction, and are transformed into a liquid state after theformation of the functionalized room temperature ionic liquid (RTIL).The RTIL is non-volatile and thus flame-resistant compared to theconventional propylene carbonate (PC) and ethyl carbonate (EC)electrolytes, which are volatile and flammable when implemented in theconventional energy storage device.

The organic salts in the embodiments of the present invention compriseacylamino group, which is a functional group having a carbon atom doublebonded with an oxygen atom, and single bonded with a nitrogen atom. Asshown in FIG. 2, the selection of the organic salts includes, but is notlimited to, acetamide, urea, methylurea (NMU), 2-oxazolidinone (OZO),ethyleneurea, and 1,3-dimethylurea DMU. The above-mentioned selectioncomprises cyclic compounds, such as OZO, ethyleneurea, and acycliccompounds, such as acetamide, urea, NMU, and DMU. The organic saltsdisclosed in the embodiments of the present invention can be readilypurchased at low cost from chemical dealers. Compared to organic salts,such as urea and acetamide, used in the conventional electrolyte, theorganic salts used in the present invention are commercially ready anddo not require any complex synthesis or purification processes, and aretherefore lower in cost.

The lithium-containing salts in the embodiments of the present inventioncomprise, but are not limited to, LiClO₄, LiSCN, LiPF₆ ⁻, LiB(C₂O₄)₂,LiN(SO₂CF₃)₂, or LiCF₃SO₃, wherein the LiN(SO₂CF₃)₂ is also known aslithium bis(trifluoromethylsulfony)imide (LiTFSI). In several preferredembodiments of the present invention, the ranges of molar ratios (ratioof lithium-containing salt to the organic salts) of the facile binarymixture are listed in the following table:

Lithium Salts:Organic Salts Range of Molar Ratios LiClO₄:acetamide1:4.2~1:5.2 LiClO₄:urea 1:3.1~1:4.1 LiClO₄:ethyleneurea 1:4.2~1:5.2LiClO₄:OZO 1:4.2~1:4.5 LiClO₄:DMU 1:4.2 LiClO₄:NMU 1:3.1~1:4.1 LiSCN:OZO1:4.2~1:6.2 LiSCN:acetamide 1:4.2~1:6.2 LiSCN:ethyleneurea 1:4.2~1:5.2LiSCN:DMU 1:4.2 LiSCN:NMU 1:3.2~1:4.2 LiTFSI:acetamide 1:4.2~1:6.2LiTFSI:urea 1:3.2~1:4.2 LiTFSI:OZO 1:3.2~1:6.2 LiTFSI:ethyleneurea 1:4.2LiPF₆:acetamide 1:4.2~1:6.2 LiPF₆:urea 1:3.2~1:4.2 LiPF₆:OZO 1:4.2~1:6.2LiPF₆:ethyleneurea 1:4.2~1:5.2

In one embodiment of the present invention, the electrolyte composed ofLiClO₄ and OZO with molar ratio 1:4.2 was prepared from LiClO₄ (AcrosInc., AP) and OZO (Acros Inc., 99%), which were dried at 110 and at 55for 12 hours in vacuum, respectively. The melting and boiling points ofOZO are 89 and 220, respectively. The content of water in theelectrolyte was determined to be less than 80 ppm with a Karl-Fischertitration. As shown in FIG. 3, the reaction presented in chemicalformula demonstrates the mixing of OZO and LiClO₄ and the formation ofthe RTIL LiClO₄:OZO. The product shows the lithium ion transferred ontothe OZO and connected through a π-bond, and the anion ClO₄ ⁻ is proximalto the cation counterpart due to the Coulomb force. The reaction ofLiClO₄:urea, which is implemented in another embodiment of the presentinvention, is also shown in FIG. 3.

The positive electrode in the embodiments of the present inventioncomprises a porous matrix having a plurality of micrometer sized poresand nanostructured metal oxides, wherein the porous matrix is a3-dimensional (3D) mesoporous metal or a 3D open-structured carbonaceousmaterial, and the nanostructured metal oxides are coated inside theplurality of pores of the porous matrix. The 3D mesoporous metalincludes, but is not limited to, nickel foam or titanium foam; whereasthe 3D open-structured carbonaceous material includes, but is notlimited to, carbon fiber papers. The nanostructured metal oxidesdisposed thereon include, but are not limited to, transitional metaloxide nanostructure, for example, MnO₂ nanowires and V₂O₅ nanosheets. Inone embodiment of the present invention, the nanostructured metal oxidesinclude MnO₂ nanoparticles, MnO₂ nanosheets, V₂O₅ nanoparticles, andV₂O₅ nanowires.

FIG. 4 shows a scanning electron microscopy (SEM) image of nickel foamthat exhibits a uniform and 3D cross-linked grid structure. The SEMimage of the nickel foam shows a pore size in a range of from 100 μm to500 μm. The electrochemical properties of the nickel foam inLiTFSI-acetamide IL and LiTFSI-urea IL were recorded with cyclicvoltammetry (CV) at a scan rate of 50 mV/s and a temperature of 303K inan argon-purified glove box, wherein both moisture and oxygen contentwere maintained below 1 ppm. A platinum wire as a reference electrodewas placed in a fitted glass tube containingN-butyl-N-methyl-purrolidinium-TFSI IL with a ferrocene/ferroceniumcouple as a potential standard (Fc/Fc⁺=50/50 mol %, potential +0.55V vs.saturated calomel electrode). A spiral platinum wire as a counterelectrode was directly immersed in the bulk LiTFSI-acetamide IL andLiTFSI-urea IL. The applied current and potential were regulated with apotentiostat. As shown in FIG. 5, a cyclic voltammogram (CV) of blanknickel foam was recorded in a) LiTFSI-acetamide IL electrolyte and b)LiTFSI-urea IL electrolyte. The potential stability windows of 6V in a)and 5V in b) are much wider than that exhibited in the conventionalaqueous solution, typically 1V. FIG. 5 shows that these ILs arepromising electrolytes for a device to store energy with a large cellvoltage.

FIG. 6 displays an SEM image of MnO₂ nanowires electrodeposited onto thesurface of the nickel foam as shown in FIG. 4. As noted, MnO₂ nanowiresare loosely dispersed on nickel foam forming a 3D and poroussuperstructure. A typical transmitted electron microscopy (TEM) image ofMnO₂ nanowires appears in FIG. 7, and shows the magnesium oxide composedof numerous interweaving nanowires having diameters of about 10 nm andlengths of up to hundreds of nanometers.

In one embodiment of the present invention, during preparation of theMnO₂ nanowires on nickel foam (MNNF) superstructure, magnesium oxide wasdeposited anodically from Mn(CH₃COO)₂ aqueous plating solution (0.25 M)at room temperature, using a three-electrode electrochemical system.Nickel foam having an area of 1 cm² was pretreated by degreasing inacetone, etching in hydrochloric acid, rinsing with water, soaking in0.01 M MnCl₂ for 4 hours, and rinsing again thoroughly with water; afterdrying, the nickel foam served as the working electrode. A platinumsheet and a saturated calomel electrode (SCE) were assembled as thecounter and reference electrodes, respectively. The electrodepositionwas performed under a constant potential 0.45V vs. SCE to give a totalpassed charge density of 0.4 Coulomb s/cm². The typical mass density ofthe deposited MnO₂, measured with a microbalance having an accuracy of0.01 mg, was approximately 0.3 mg cm⁻². In other embodiments of thepresent invention, the above-mentioned preparation process can also beapplied to other nanostructured metal oxide-coated metal foams withdifferent electrodepositing potentials.

The electrochemical properties of the MNNF in various electrolytes wererecorded with cyclic voltammetry (CV). A platinum wire as a referenceelectrode was placed in a fitted glass tube containingN-butyl-N-methyl-purrolidinium-TFSI IL with a ferrocene/ferroceniumcouple as a potential standard (Fc/Fc⁺=50/50 mol %, potential +0.55V vs.SCE). A spiral platinum wire as a counter electrode was directlyimmersed in the ILs. In FIG. 8, three cyclic voltammograms of an MNNF inthe a) aqueous electrolyte (498 F g⁻¹, 0.9 V), b) LiClO₄-urea IL (364 Fg⁻¹, 2.5 V), and c) LiClO₄-OZO IL (225 F g⁻¹, 3 V) are shown.Irreversible cathodic and anodic reactions were observed at differentranges of the operating potential. The operating potential region ofabout 0.9V is presented for the aqueous electrolyte; 2.5V for theLiClO₄-OZO IL electrolyte; and 3.0V for the LiClO₄-urea IL electrolyte.However, there is room to improve the performance in terms of energy,power densities, safety, and cycle life of the state-of-the-artelectrochemical capacitors. The energy density (E) and power density (P)of an electrochemical capacitor are governed by the following equations:E=(C(ΔV)²)/2  (1)P=E/t,   (2)where C is capacitance, Δ V is cell voltage, and t is the dischargingtime. The applicable potential window of manganese oxide (MnO₂) in IL,approximately 2.5 V˜3 V, is three times more than that found intraditional aqueous electrolytes. This allows the ECs to operate at ahigher cell voltage, and therefore, the energy and power densities areimproved. The quasi-rectangular response of the CV shape is attributedto a continuous and reversible Mn³⁺/Mn⁴⁺ faradaic redox reaction of MnO₂nanowires over the potential ranges of different electrolytes.

Embodiment 1

One embodiment of the chemical energy storage system in the presentinvention comprises a positive electrode composed of MNNF; a negativeelectrode composed of inert graphite disposed proximal to, but not incontact with, the MNNF electrode; a non-aqueous electrolyte includingOZO and LiClO₄; and a case accommodating the non-aqueous electrolyte,the positive electrode, and the negative electrode. The preparation ofthe MNNF electrode is described above, and includes application of aconstant potential of 0.45V vs. SCE during the electrodepositionprocess.

The electrochemical properties of the MNNF electrode in LiClO₄-OZO ILwere recorded with a chronopotentiogram (CP). The obtained CP of theMNNF electrode for charge-discharge cycles in an applied potential from−1.5V to +1.0V and an applied current density of ∓1.5 A g⁻¹ is shown inFIG. 9. The MNNF electrode exhibits enduring electrochemical stability,that is, the charge/discharge cycling induces a barely noticeabledegradation of its 3D porous superstructure. The high electrodeoperating potential 2.5V in the LiClO₄-OZO IL allows a superior energydensity of 304 W h kg⁻¹ of the capacitor because the energy densityrelates mainly to the square of the cell operating voltage.

Embodiment 2

Another embodiment of the chemical energy storage system in the presentinvention comprises a positive electrode composed of V₂O₅ nanosheets andtitanium foam; a negative electrode composed of inert graphite disposedproximal to, but not in contact with, the positive electrode; anon-aqueous electrolyte including acetamide and LiClO₄; and a caseaccommodating the non-aqueous electrolyte, the positive electrode, andthe negative electrode. The preparation of the V₂O₅ nanosheets-coatedtitanium electrodes are described above, and includes application of aconstant potential of 0.65V vs. SCE during the electrodepositionprocess.

The electrochemical properties of the V₂O₅ nanosheets-coated titaniumelectrode in LiClO₄-acetamide IL were recorded with a chronopotentiogram(CP). The obtained CP of the V₂O₅ nanosheets-coated titanium electrodefor charge-discharge cycles in an applied potential from −2.1V to +2.1Vand an applied current density of ∓1.0 A g⁻¹ is shown in FIG. 10. Thespecific capacitance, measured at a sweep rate of 10 mV/s, was 240 F/g.The high electrode operating potential 4.2V in the LiClO₄-acetamide ILallows a superior energy density of 560 W h kg⁻¹ of the capacitorbecause the energy density relates mainly to the square of the celloperating voltage.

Embodiment 3

Another embodiment of the chemical energy storage system in the presentinvention comprises a positive electrode composed of MnO₂ nanowires andcarbon fiber paper; a negative electrode composed of inert graphitedisposed proximal to, but not in contact with, the positive electrode; anon-aqueous electrolyte including urea and LiClO₄; and a caseaccommodating the non-aqueous electrolyte, the positive electrode, andthe negative electrode. The preparation of the MnO₂ nanowire-coatedcarbon fiber paper electrodes is described above, and includesapplication of a constant potential of 1 mAcm⁻² vs. SCE during theelectrodeposition process.

The electrochemical properties of the MnO₂ nanowire-coated carbon fiberpaper electrode in LiClO₄-urea IL were recorded with achronopotentiogram (CP). The obtained CP of the MnO₂ nanowire-coatedcarbon fiber paper electrode for charge-discharge cycles in an appliedpotential from −1.8V to +1.2V and an applied current density of ∓1.0 Ag⁻¹ is shown in FIG. 11. The specific capacitance, measured at a sweeprate of 10 mV/s, was 250 F g⁻¹. The high electrode operating potential3.0V in the LiClO₄-urea IL allows a superior energy density of 315 W hkg⁻¹ of the capacitor because the energy density is related mainly tothe square of the cell operating voltage.

FIG. 12 shows an electrochemical energy storage system 120 having asealed container 125 in one embodiment of the present invention. Thesealed container 125 has an inner surface 125 a contacting thenon-aqueous electrolyte 124 and an outer surface 125 b not contactingthe electrolyte 124. A positive contact 128 and a negative contact 127are allowed to be either positioned at different regions of the outersurface 125 b or extended out from the container 125 and reaching theobject to be charged. The positive contact 128 is electrically connectedwith the positive electrode 121, and the negative contact 127 iselectrically connected with the negative electrode 122. Since the sealedcontainer possesses a compact nature, a separation layer 123 may furtherbe positioned between the positive electrode 121 and the negativeelectrode 122, so as to prevent two electrodes from physicallyconnecting with each other. The composition of the electrolyte 124, thepositive electrode 121, and the negative electrode 122 can be selectedfrom the materials described in the previous embodiments.

The performance of the chemical energy storage systems described in thepresent invention opens a new route for development of efficient systemsto store energy using metal-oxide electrodes deposited on porousmaterials with ionic liquid electrolytes. If the results can besuccessfully scaled to a thickness of hundreds of microns without lossof performance, an MNNF electrode in LiClO₄-OZO IL electrolyte-based ECmight be promising for applications in electric vehicles and heavymachinery that require rapid and high power energy storage systems.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A non-aqueous electrolyte for an electrochemicalcapacitor, comprising: organic compounds having at least one acylaminogroup; and lithium salts characterized as LiX, wherein Li is lithium andX comprises SCN⁻; the organic compounds and the lithium salts are insolid state before mixing; the organic compounds are cyclic compounds;and the cyclic compounds comprise 2-oxazolidinone, ethyleneurea, or thecombination thereof.
 2. The non-aqueous electrolyte of claim 1, whereinthe lithium salts and the organic compounds are mixed with molar ratiosin a range of from 1:3 to 1:7, and wherein the molar ratios are theratios of lithium salts to organic compounds in moles.